The present invention relates to a rubber composition or, more particularly, to a highly flame-retardant rubber composition based on a copolymer of ethylene and propylene emitting a remarkably small volume of smoke even when it is set on fire.
Various kinds of synthetic rubbers based on a copolymer of monomers comprising ethylene and propylene, referred to generally as EP rubbers hereinbelow, including copolymers of ethylene and propylene, referred to as EPM rubbers hereinbelow, and copolymers of ethylene, propylene and a diene monomer, referred to as EPDM rubbers hereinbelow, are known and widely used as an electric insulating material by virtue of their excellent heat resistance, weatherability and electric properties. Since these rubbery polymers are inherently inflammable, it is usual in the application thereof to an electric insulation use that the rubber is compounded with a large amount of a halogen-containing compound as a flame retardant.
The addition of such halogen-containing flame retardants is of course very detrimental to the heat resistance and mechanical properties of the rubber composition inherent in the EPM and EPDM rubbers. Moreover, a rubber composition compounded with a large amount of a halogen-containing flame retardant is very disadvantageous from the standpoint of safety, health of human body and environmental pollution when it is set on fire because combustion of such a rubber composition necessarily produces a large volume of toxic or corrosivce gas as well as a large volume of smoke.
An alternative flame retardant of the above mentioned halogen-containing compounds is red phosphorus although the effect of flame retardancy obtained thereby is relatively low in comparison with halogen-containing compounds in addition to the still larger adverse effects on the heat resistance and mechanical properties of the rubber composition than halogen-containing compounds. Moreover, the improvement in the smoke emission is also insufficient by compounding the rubber with red phosphorus.